The basic concept of thermodynamically coupling a Rankine-cycle steam turbine and a Brayton-cycle gas turbine to achieve overall efficiency higher than would be obtainable for either cycle alone (working in the same range of fluid temperatures and pressures) is well known. The higher efficiency achieved by combining cycles reduces fuel requirements and costs, and conserves energy that otherwise would be rejected to the environment as heat. The quantity and type of undesirable effluents in the plant exhaust gases can also be limited if a combined cycle plant is restricted to burning natural gas or refined or specially treated petroleum oils. Unfortunately, at the present time, these clean burning fuels are relatively scarce as compared to carbonaceous solid fuels such as coal, and alternative means have been sought to make coal, regardless of its rank or quality, an acceptable fuel for combined cycle power plants.
An important development in the field of power generation technology has been the use of coal-fueled sulfur-sorbing fluidized bed combustors to provide the basic energy source while minimizing power plant emissions. Although burning coal, it has been found that environmentally safe, sulfur-sorbing fluidized bed combustors can be utilized in combined cycle power plants to provide a gas turbine motive fluid that is free from undue corrosion and fouling effects which otherwise would cause severe reliability problems. Such uses are disclosed in U.S. Pat. No. 4,116,005 to Willyoung, and in the allowed U.S. patent application of Willyoung filed concurrently herewith under application Ser. No. 063,469 and assigned to the assignee of the present invention.
With these coal burning combined cycle plants, however, limitations of the sulfur-sorbing fluidized bed combustor have constrained the gas turbine to operate at less than state-of-the-art inlet temperatures, resulting in the gas turbine making less than optimal contribution to overall plant efficiency. Present-day gas turbines are able to operate at firing temperatures much higher than can now be obtained with a fluidized bed heater. The temperature discrepancy (which ma be as much as 450.degree. F. for current commercial gas turbines and 800.degree. F. or more for gas turbine designs projected for the future) is caused by two operational limitations on the fluidized bed combustor. First, because coal contains significant levels of sulfur, the bed temperature must be limited to a range which will insure sufficient sulfur capture by reaction with the in-bed sorption material to satisfy air emission standards. The second constraint that acts to limit fluidized bed combustor temperatures arises from the limitations and capabilities of presently available metallic materials which are suitable for in-bed heat transfer surfaces or for meeting structural requirements.
It is one object of the present invention, therefore, to provide an improved combined cycle power plant having a coal-fueled fluidized bed combustor as its principal source of energy and wherein the gas turbine is operated at state-of-the-art levels of specific power and firing temperature so that the highest overall power plant efficiency is attained.
Another object of the invention is to significantly increase the overall efficiency of a combined cycle steam turbine and gas turbine power plant without using a large proportion of premium fuel such as natural gas or petroleum.
Yet another object of the invention is to provide a highly efficient combined cycle power plant including a reliable gas turbine not exposed to coal combustion products.